36 VARIATION 



Bilateral symmetry not complete. Curiously enough not all the 

 parts follow the same plan as to bilateralism and symmetry. What 

 has been said refers to paired organs standing on opposite sides 

 of the body, as hands, arms, legs, eyes, ears, etc. 



Many organs not paired present curious facts to the evolu- 

 tionist. The nose, for example, has no counterpart, but it stands 

 on the median line and has a bilateral symmetry of its own, 

 being made up of right and left halves. The liver and the heart, 

 however, while consisting of right and left halves, are impaired 

 organs, placed not on the median line, but the one upon the right, 

 the other upon the left. Each has a bilateral symmetry of its 

 own, with distinct right and left sides, yet both are unsymmetric- 

 ally placed. 



The stomach, on the other hand, is an unpaired organ lying 

 unsymmetrically across the body, and its own bilateralism is not 

 between right and left but from front to back. The kidneys pre- 

 sent the anomalous phenomena of paired organs with a bilateral- 

 ism of their own but at right angles to that of the body, being 

 also from front to back. 



Longitudinal symmetry and linear series. Inasmuch as all 

 growth is by cell division, we might expect longitudinal symmetry 

 as well as lateral symmetry. Owing, however, to the definite 

 relations of both animals and plants to the external world, it is 

 not much developed, and there is but a suggestion of longitudinal 

 symmetry to be found in either plant or animal forms. 



Most plants, are both geotropic and heliotropic ; that is, one 

 part goes down into the earth in response to gravity and the 

 other upward toward the light and against gravity. This makes 



common descent. Thus the leg of a man is homologous with that of a horse or a 

 bird, because of structural resemblances. In the same sense his arm is homolo- 

 gous with the fore leg of the horse or the wing of a bird. 



Analogous parts are such as serve the same purpose in different organisms 

 though structurally distinct. Thus the flipper of the whale, which is a modified 

 hand, is analogous to the fin of a fish, and the gill of a fish is analogous to the 

 lung of a mammal, because it serves the same purpose, though there is no struc- 

 tural relation between the two. 



The homologue or the analogue of a part is therefore to be found in another 

 individual and of a different species. Symmetry, on the contrary, with its corollary 

 of multiple parts, refers to individuals taken singly and to the interrelations of 

 their parts. 



